The rapid development of nitrides emitting devices in recent years has resulted in high demands in high brightness LED. Along with the significant enhancement in emission efficiency, high interest and expectation are given to the use of high brightness LED to serve as a white light source. Presently, the structures using high brightness LED to serve as white light sources include the followings:
The first structure is developed by Nichia Kagaku Kogyo Kabushiki Kaisha, where yellow fluorescent powder (YAG: Ce; Y3Al5O12: Ce3+) is added into blue LED to generate mixed white light (with reference to U.S. Pat. No. 6,069,440). Such a structure provides a white light emitting device that is of the lowest cost among the commercially available white light emitting devices. However, with the blue halo effect, the reducing reliability and low light conversion efficiency of fluorescent powder, and the limited characteristics of fluorescent powder, such a structure cannot attain white light emitting devices with high color saturation, high luminous intensity, high reliability and capable of various color temperature modulation.
The second structure was recently developed and aimed at improving the poor color rendering index (CRI) of the aforementioned white light source, where ultraviolet A (UVA) was excited to generate red, green and blue fluorescent powder, for producing white light sources with a high CRI (with reference to U.S. Pat. Nos. 6,592,780; 6,580,097 and 6,596,195). Such a structure involves the drawbacks of poor reliability in the mixture of RGB fluorescent powder. In addition, the light source generated from exciting UVA cannot serve as a mixed light source, thereby resulting in an even lower luminous intensity. Furthermore, such a structure further needs to overcome the safety concerns of resin deterioration and UVA leakage during the packaging process.
The third structure is one of a high cost structure, which joins a plurality of emitting devices to generate high brightness thereby attaining a white light source with an excellent CRI (with reference to U.S. Pat. No. 6,563,139). However, the commercially available package attained from such a structure is limited to that including red-orange-yellow light AlGaInP LED and nitrides blue LED with a wavelength greater than 580 nm. As such, the drawbacks of such a structure include the high cost involved in packaging multiple chips, and difficulty involved in packaging the two LED of different characteristics, such as thermal stability, driving voltages and material reliability.
In addition, others also suggest the use of plural quantum wells with different wave bands in a single chip to directly generate white light (with reference to JP2001-028458). Since the use of quantum wells to serve as an AlGaInP emitting layer cannot attain wave bands with high emission efficiency, such as a wavelength greater than 550 nm, and the emission bandwidth is relative to the emission efficiency. Thus, the drawback of significantly reduced emission efficiency of the quantum well emitting layer will be observed upon increasing its emission bandwidth. Thus, quantum well emitting layer cannot only be used to attain good emitting efficiency under a narrow-bandwidth wavelength. Accordingly, the process for manufacturing such a device and the emission efficiency of such a device cannot meet the performance demands of commercial white light that requires high intensity, a high CRI, and is capable of color temperature modulation.
A further alternative is to use an Al(1−x−y)InyGaxN blue light chip to excite AlGaInP for generating yellow light that is then mixed to generate a white light source. However, the low intensity and narrow bandwidth of the yellow light result in poor lighting effects. Yet another alterative is to use ZnSe as the luminescent material (with reference to U.S. Pat. No. 6,337,536). However, the reliability, color saturation, luminous intensity of such is inferior to white light emitting devices of the Al(1−x−y)InyGaxN type.
Thus, there has been a need for a novel white light emitting device capable of resolving the above drawbacks.